Optimization of fluoride removal using calcined bauxite: Adsorption isotherms and kinetics

Abstract

Geogenic contamination of groundwater due to elevated fluoride (F−) concentrations is a significant issue worldwide (including in Tanzania). The present study focussed to assess the adsorption capacity of thermally treated (calcined) bauxite to remove the F− from contaminated water. Characterization of bauxite by X-ray fluorescence spectroscopy (XRF) revealed Al2O3, Fe2O3, and SiO2 as the major oxides in both raw and calcined bauxite. The major mineral phase in the raw bauxite was gibbsite, which disappeared after calcination. The optimum calcination temperature, dosage and contact time for F− removal by calcined bauxite were 400 °C, 40 g/L and 8 min, respectively. The experimental data revealed Freundlich isotherm as the best model to fit the F− adsorption process with kF and 1/n being 0.1537 mg/g and 0.8607, respectively. The pseudo-second-order kinetic and intra-particle diffusion models explained well the F− adsorption process with the rate constants of 115.43 g/mg min and 0.0025 mg/g min0.5, respectively. The values of ΔG, ΔH and ΔS indicate the F− adsorption on bauxite surface indicated that the adsorption process was spontaneous, endothermic and structural changes occurred during the adsorption process. The F− adsorption under optimum conditions lowered the pH and F− concentration to WHO and Tanzania Bureau of Standards (TBS) standards.